EP3439201B1

EP3439201B1 - Signal loopback circuit and signal loopback method

Info

Publication number: EP3439201B1
Application number: EP17774545.2A
Authority: EP
Inventors: Satoshi Mikami
Original assignee: NEC Corp
Current assignee: NEC Corp
Priority date: 2016-03-30
Filing date: 2017-03-22
Publication date: 2022-03-16
Anticipated expiration: 2037-03-22
Also published as: EP3439201A1; WO2017170004A1; JPWO2017170004A1; EP3439201A4; US20190097727A1; CN109075857A; CN109075857B; US10587343B2; JP6699721B2

Description

Technical Field

The present invention relates to a signal loopback circuit and a signal loopback method that are used in an optical submarine cable system.

Background Art

FIG. 7 is a block diagram illustrating a configuration example of a relay device 900 related to the present invention. The relay device 900 is a repeater (optical repeater) for an optical signal, which is used in an optical submarine cable system. The relay device 900 includes optical amplifiers 901 and 902. The optical amplifiers 901 and 902 are designed in such a way as to be suitable for amplification of a C-band optical signal. Note that in the present patent application, the term "C-band" indicates a wavelength band approximately ranging from 1530 nm to 1565 nm.
From an uplink input (UP IN) in FIG. 7, an uplink-channel optical signal whose carrier wavelength is within the C-band is wavelength-multiplexed and input. The input uplink-channel optical signal is amplified by the optical amplifier 901. The uplink optical signal amplified by the optical amplifier 901 is output from an uplink output (UP OUT). From a downlink input (DOWN IN) in FIG. 7, a downlink-channel optical signal whose carrier wavelength is within the C-band is wavelength-multiplexed and input. The input downlink-channel optical signal is amplified by the optical amplifier 902. The optical signal amplified by the optical amplifier 902 is output from a downlink output (DOWN OUT). Optical signals input and output in the relay device 900 are transmitted between the relay device 900 and a land device or another relay device.
At the outputs of the optical amplifiers 901 and 902, signal loopback circuits 903 and 904 are provided respectively. The signal loopback circuits 903 and 904 are each constituted of an optical fiber grating and two optical couplers. The signal loopback circuits 903 and 904 reflect monitoring signals among optical signals output from the optical amplifiers 901 and 902 respectively, and loop back the reflected signals to channels in the opposite directions. The monitoring signal is a signal for monitoring a system and has a wavelength different from that of data to be transmitted. The monitoring signals looped back by the signal loopback circuits 903 and 904 are looped back to land devices by using opposite optical channels. The land device can remotely monitor an operation of the relay device 900 by using the looped-back monitoring signal. For example, a downlink-channel monitoring signal included in an optical signal output from the optical amplifier 901 is combined to an uplink-channel optical signal by the signal loopback circuit 904. As a result, the land device connected to the uplink output (UP OUT) can receive the downlink-channel monitoring signal.
In relation to the present invention, PTL 1 describes an optical amplification relay system including an optical loopback circuit for a monitoring signal. Prior art documents PTL 2 - PTL 5 describe further state-of-the art loopback circuits.

Citation List

Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. 2002-280968
PTL 2: EP 0 981 215 A2
PTL 3: US 2014/086573 A1
PTL 4: US 2009/324249 A1
PTL 5: EP 1037 409 A2

Summary of Invention

Technical Problem

In order that a capacity of an optical submarine cable system is made large, an optical relay device capable of amplifying an L-band optical signal, in addition to a C-band optical signal widely used up to the present, has been studied. However, the relay device 900 illustrated in FIG. 7 and the invention described in PTL 1 do not include a configuration for looping back a monitoring signal in a system in which an optical signal having a plurality of wavelength bands (i.e., the C-band and the L-band) is transmitted. Note that in the present patent application, the term "L-band" indicates a wavelength band approximately ranging from 1570 nm to 1610 nm.

(Object of Invention)

An object of the present invention is to provide a technique for looping back a monitoring signal in a relay device that relays an optical signal having a plurality of wavelength bands.

Solution to Problem

A signal loopback circuit according to the present invention is a signal loopback circuit for making connection between a first channel of a first direction and a second channel of a second direction, the first direction being different from the second direction, wherein an optical signal of a first wavelength band and an optical signal of a second wavelength band are configured to be transmitted through each of the first and second channels, the second wavelength band being different from the first wavelength band, the signal loopback circuit comprising: a first coupler (141) configured to split the optical signal of the first wavelength band in the first channel of the first direction; a first filter (142) in the first channel of the first direction and configured to extract, from the optical signal of the first wavelength band split by the first coupler (141), a monitoring signal of the first wavelength band; a second coupler (143) configured to combine, to the second channel of the second direction, the monitoring signal of the first wavelength band extracted by the first filter (142); a third coupler (151) configured to split the optical signal of the second wavelength band in the first channel of the first direction; a second filter (152) in the first channel of the first direction and configured to extract, from the optical signal of the second wavelength band split by the third coupler (151), a monitoring signal of the second wavelength band; and a fourth coupler (153) configured to combine, to the second channel of the second direction, the monitoring signal of the second wavelength band extracted by the second filter (152).
A signal loopback method according to the present invention is a signal loopback method for making connection between a first channel of a first direction and a second channel of a second direction, the first direction being different from the second direction, wherein an optical signal of a first wavelength band and an optical signal of a second wavelength band are transmitted through each of the first and second channels, the second wavelength band being different from the first wavelength band, the signal loopback method comprising: splitting the optical signal of the first wavelength in the first channel of the first direction; extracting, from the split optical signal of the first wavelength, a monitoring signal of the first wavelength band that is used in the first channel of the first direction; and combining the extracted monitoring signal of the first wavelength to the second channel of the second direction; splitting the optical signal of the second wavelength in the first channel of the first direction; extracting, from the split optical signal of the second wavelength, a monitoring signal of the second wavelength band that is used in the first channel of the first direction; and combining the extracted monitoring signal of the second wavelength to the second channel of the second direction.

Advantageous Effects of Invention

In a relay device that relays an optical signal having a plurality of wavelength bands, a monitoring signal can be looped back.

Brief Description of Drawings

FIG. 1 is a block diagram illustrating a configuration example of a relay device 100 according to a first example embodiment.
FIG. 2 is a diagram illustrating an example of wavelengths of an optical signal passing through the relay device 100.
FIG. 3 is a block diagram illustrating a configuration example of a relay device 200 according to a second example embodiment.
FIG. 4 is a block diagram illustrating a configuration example of a relay device 300 according to a third example embodiment.
FIG. 5 is a block diagram illustrating a configuration example of a relay device 400 according to a fourth example embodiment.
FIG. 6 is a block diagram illustrating a configuration example of a relay device 500 according to a fifth example embodiment.
FIG. 7 is a block diagram illustrating a configuration example of a relay device 900 related to the present invention.

Example Embodiment

FIG. 1 is a block diagram illustrating a configuration example of a relay device 100 according to a first example embodiment of the present invention. The relay device 100 is, for example, a submarine repeater used in an optical submarine cable system. In the drawings mentioned below, the arrows attached to signals are given for illustration of an operation example, and do not limit types and directions of the signals.
To the relay device 100, four submarine cables are connected. "UP IN" is an input of an uplink signal, "UP OUT" is an output of an uplink signal, "DOWN IN" is the input of a downlink signal, and "DOWN OUT" is the output of a downlink signal. The relay device 100 has a relay function of amplifying bidirectional optical signals in the uplink direction and the downlink direction. The relay device 100 includes a multiplexing-demultiplexing devices 111 to 114, optical amplifiers 121 to 124, and signal loopback circuits 131 to 134. The multiplexing-demultiplexing devices 111 to 114 are, for example, optical wavelength filters, and multiplex and demultiplex optical signals of wavelength bands in the C-band and the L-band. A wavelength of an optical signal passing through the signal loopback circuits 131 to 134 is described with reference to FIG. 2.
The optical amplifiers 121 and 123 are optical fiber amplifiers designed in such a way as to be suitable for amplification of a C-band optical signal. The optical amplifiers 122 and 124 are optical fiber amplifiers designed in such a way as to be suitable for amplification of an L-band optical signal.
An uplink optical signal input from the UP IN is a wavelength-multiplexed signal including optical signals of C-band and L-band. In the multiplexing-demultiplexing device 111, the uplink optical signal is separated into the C-band optical signal and the L-band optical signal. The C-band optical signal is amplified by the optical amplifier 121. The L-band optical signal is amplified by the optical amplifier 122. The optical signals amplified by the optical amplifiers 121 and 122 are wavelength-multiplexed by the multiplexing-demultiplexing device 112, and are output from the UP OUT. The UP OUT is connected to another adjacent relay device or a land device. In the procedure similar to the optical signal in the uplink direction, an optical signal in the downlink direction input from the DOWN IN is also amplified by the optical amplifier 123 or the optical amplifier 124, and is output from the DOWN OUT.
Each of the signal loopback circuits 131 to 134 has a function of reflecting an optical signal with a part of wavelengths of an optical signal output from the optical amplifiers 121 to 124 and looping back the reflected signal by using the channel in the opposite direction. Each of the signal loopback circuits 131 to 134 includes a reflecting element and two optical couplers.
A configuration and an operation of the signal loopback circuits 131 to 134 are described by citing the signal loopback circuit 131 as an example. The signal loopback circuit 131 includes couplers 141 and 143, and a reflecting element 142. Optical directional couplers can be used for the couplers 141 and 143. An optical fiber grating can be used for the reflection element 142. An optical signal output from the optical amplifier 121 is split by the coupler 141 of the signal loopback circuit 131, and is made incident on the reflecting element 142. The reflecting element 142 reflects only an optical signal having a part of wavelengths of the incident optical signal. Specifically, the reflecting element 142 reflects only an optical signal having a wavelength of a C-band monitoring signal of the uplink channel. In other words, the reflecting element 142 extracts the C-band monitoring signal of the uplink channel.
The reflected monitoring signal is input to the optical amplifier 123 for the downlink direction, via the couplers 141 and 143. Since the optical amplifier 123 is a C-band amplifier, the monitoring signal of the uplink channel as well as an optical signal of the downlink channel is amplified by the optical amplifier 123.
In this manner, the signal loopback circuit 131 can loop back, to the downlink transmission path, an optical signal (i.e., a monitoring signal) having a part of wavelengths of a C-band uplink optical signal. By reflecting the monitoring signal in the C-band uplink signal by the signal loopback circuit 131, the monitoring signal of the uplink channel can be transmitted to a land device on a path of the downlink channel. As a result, at the land device connected to the downlink channel, the uplink channel of the relay device 100 can be remotely monitored.
The relay device 100 illustrated in FIG. 1 includes signal loopback circuits respectively in the transmission paths for wavelength bands of both of the C-band and L-band. In other words, the signal loopback circuit 131 is used for looping back a C-band monitoring signal of the uplink, and the signal loopback circuit 132 is used for looping back an L-band monitoring signal of the uplink. The signal loopback circuit 132 is constituted by couplers 151 and 153 and a reflecting element 152. Optical directional couplers can be used for the couplers 151 and 153. An optical fiber grating can be used for the reflecting element 152. An optical signal output from the optical amplifier 122 is split by the coupler 151 of the signal loopback circuit 132, and is made incident on the reflecting element 152. The reflecting element 152 reflects only an optical signal having a part of wavelength of the incident optical signal. Specifically, the reflecting element 152 reflects only the optical signal having the wavelength of the L-band monitoring signal of the uplink channel. The reflected monitoring signal is input to the optical amplifier 124 for the downlink direction via the couplers 151 and 153. Similarly, the signal loopback circuit 133 is used for looping back a C-band monitoring signal of the downlink, and the signal loopback circuit 134 is used for looping back an L-band monitoring signal of the downlink.
With such a configuration, a monitoring signal is transmitted to a land device connected to the channel in the opposite direction. As a result, an uplink-channel land device connected with the uplink output (UP OUT) can monitor the downlink channel for both of the C-band and the L-band. Further, a downlink-channel land device connected with the downlink output (DOWN OUT) can monitor the uplink channel for both of the C-band and the L-band.
FIG. 2 is a diagram illustrating an example of wavelengths of an optical signal passing through the relay device 100. The uplink signal indicated by "UP" in FIG. 2 includes wavelength-multiplexed signals of wavelengths λ1 to λm and wavelengths λm+1 to λn, and monitoring signals of wavelength λsvC-U and wavelength λsvL-U. The downlink signal indicated by DOWN in FIG. 2 includes wavelength-multiplexed signals of wavelengths λ1 to λm and wavelengths λm+1 to λn, and monitoring signals of wavelengths λsvC-D and λsvL-D. The symbols m and n are natural numbers satisfying m<n. The optical signal of the wavelengths λ1 to λm (generically referred to as λC) is a main signal of the C-band, and the optical signal of the wavelengths λm+1 to λn (generically referred to as λL) is a main signal of the L-band. In other words, the main signal of the C-band has been multiplexed with m wavelengths at the maximum, and the main signal of the L-band has been multiplexed with n-m wavelengths at the maximum.
The monitoring signals of wavelengths λsvC-U and λsvL-U are a C-band monitoring signal of the uplink channel and an L-band monitoring signal of the uplink channel, respectively. The monitoring signals of the wavelengths λsvC-D and λsvL-D are the C-band monitoring signal of the downlink channel and the L-band monitoring signal of the downlink channel, respectively. The wavelengths λsvC-U, λsvL-U, λsvC-D, and λsvL-D of the monitoring signals are different from each other, and do not overlap with the wavelengths of the main signals. Further, each of the monitoring signals may include signals of a plurality of wavelengths.
The relay device 100 with such a configuration can relay an optical signal having a plurality of wavelength bands, and can loop back a monitoring signal. In particular, the relay device 100 includes the signal loopback circuit for each of the C-band optical signal and the L-band optical signal, and thereby, can loop back monitoring signals of the L-band as well as the C-band. Further, since the looped-back monitoring signals are multiplexed on the input side of one of the optical amplifiers 121 to 124, the monitoring signals are amplified by the optical amplifier and then output. As a result, an optical level of the monitoring signal output from the relay device 100 is raised, and monitoring sensitivity at a land device is improved.

Second Example Embodiment

FIG. 3 is a block diagram illustrating a configuration example of a relay device 200 according to a second example embodiment of the present invention. Similarly to the relay device 100 according to the first example embodiment, the relay device 200 is a repeater used in an optical submarine cable system. In the following description of the example embodiment, the same reference symbols are attached to the elements described above, and the overlapping description is appropriately omitted.
In comparison with the relay device according to the first example embodiment, the relay device 200 includes signal loopback circuits 211 and 212 instead of the signal loopback circuits 131 to 134. The signal loopback circuits 211 and 212 each have a function of looping back, to the channel in the opposite direction, an optical signal with a part of wavelengths of an optical signal output from the multiplexing-demultiplexing devices 112 and 114. Each of the signal loopback circuits 211 and 212 includes two reflecting elements and two optical couplers.
The function of the signal loopback circuits 211 and 212 is described by citing the signal loopback circuit 211 as an example. The signal loopback circuit 211 is constituted by couplers 241, 243 and a reflecting element 242. Optical directional couplers can be used for the couplers 241 and 243. Two optical fiber gratings connected in series can be used for the reflecting element 242. The multiplexing-demultiplexing device 112 wavelength-multiplexes and outputs a C-band optical signal and an L-band optical signal. The wavelength-multiplexed optical signal is split by the coupler 241 of the signal loopback circuit 211, and is made incident on the reflecting element 242. The reflecting element 242 includes two fiber gratings whose reflection wavelengths are different from each other. Thus, two optical signals whose wavelengths are different from each other are reflected at the reflecting element 242. The reflected optical signals are input to the multiplexing-demultiplexing device 113 in the downlink direction via the couplers 241 and 243. The optical signals reflected by the reflection element 242 are amplified by the optical amplifier 123 or 124 corresponding to the wavelengths thereof, are multiplexed by the multiplexing-demultiplexing device 114, and are output from the DOWN OUT. In this manner, the signal loopback circuit 211 can loop back, to the downlink channel, the optical signal having a part of wavelengths of the uplink optical signals of the C-band and the L-band.
The signal loopback circuit 211 can loop back, to the downlink channel, a C-band monitoring signal (a wavelength λsvC-U) of the uplink channel and an L-band monitoring signal (a wavelength XsvL-U) of the uplink channel. As a result, the uplink-channel monitoring signals of both the C-band and the L-band are transmitted to a land device connected to the downlink channel, and in the land device, the uplink channel of the relay device 200 can be remotely monitored. Similarly, since the signal loopback circuit 212 can loop back downlink-channel monitoring signals (wavelengths λsvC-D and λsvL-D) of both the C-band and the L-band, the downlink channel of the relay device 200 can be remotely monitored in a land device connected to the uplink channel.
The relay device 200 according to the second example embodiment can relay an optical signal having a plurality of wavelength bands, and can loop back monitoring signals. In particular, by one signal loopback circuit 211 or 212, the relay device 200 loops back the optical signal multiplexed by the multiplexing-demultiplexing device 112 or 114, and thus, has a configuration simplified as compared with the relay device 100 according to the first example embodiment. Further, similarly to the relay device 100 according to the first example embodiment, in the relay device 200, the looped-back monitoring signals are multiplexed on the input side of the optical amplifier, and thus, the monitoring signals are amplified by the optical amplifier and output. As a result, an optical level of the monitoring signal is raised, and monitoring sensitivity at a land device is improved.

Third Example Embodiment

FIG. 4 is a block diagram illustrating a configuration example of a relay device 300 according to a third example embodiment of the present invention. Similarly to the relay device 200 according to the second example embodiment, the relay device 300 is a submarine repeater used in an optical submarine cable system.
In comparison with the relay device 200 according to the second example embodiment, the relay device 300 includes a signal loopback circuit 311 instead of the signal loopback circuits 211 and 212. The signal loopback circuit 311 has a function of, by using the optical transmission path in the opposite direction, looping back an optical signal with a part of wavelengths of an optical signal output from the multiplexing-demultiplexing devices 112 and 114. The signal loopback circuit 311 includes two pairs of reflecting elements and two optical couplers.
The function of the signal loopback circuit 311 is described. The signal loopback circuit 311 includes couplers 312 and 314, and reflecting elements 313 and 315. Optical directional couplers can be used for the couplers 312 and 314. Two optical fiber gratings connected in series can be used for the reflecting elements 313 and 315, respectively.
An uplink-channel optical signal output from the multiplexing-demultiplexing device 112 is split by the coupler 312 of the signal loopback circuit 311, and is made incident on the reflecting element 313. The reflecting element 313 reflects only an optical signal having a part of wavelengths of the incident optical signal. The reflecting element 313 includes, for example, two fiber gratings whose reflection wavelengths are different from each other. Thus, an optical signal having at least two wavelengths is reflected at the reflecting element 313. The reflected optical signal is combined to the downlink channel via couplers 312 and 314, and is output from the DOWN OUT. In this manner, the signal loopback circuit 311 can loop back, to the downlink transmission path, the optical signal having a part of wavelengths of the uplink-channel optical signal of the C-band and L-band.
By reflecting uplink monitoring signals (wavelengths λsvC-U and λsvL-U) of the C-band and the L-band by the reflecting element 313, uplink-channel monitoring signals of both the C-band and the L-band can be transmitted, by using the downlink channel, to a land device connected to the downlink channel. As a result, the uplink channel of the relay device 300 can be remotely monitored in a land device connected to the downlink channel. Similarly, the signal loopback circuit 311 reflects downlink monitoring signals (wavelengths λsvC-D and λsvL-D) of the C-band and the L-band by the reflecting element 315, and thereby, can loop back, to the uplink channel, downlink monitoring signals of both the C-band and the L-band.
The relay device 300 according to the third example embodiment can relay an optical signal having a plurality of wavelength bands, and can loop back uplink monitoring signals of both the C-band and the L-band, at the signal loopback circuit 311. Further, the relay device 300 loops back monitoring signals of both the uplink channel and the downlink channel by one signal loopback circuit 311, and thus, has a configuration further simplified as compared with the relay device 200.

Fourth Example Embodiment

FIG. 5 is a block diagram illustrating a configuration example of a relay device 400 according to a fourth example embodiment of the present invention. Similarly to the relay device 200 according to the second example embodiment, the relay device 400 is a submarine repeater used in an optical submarine cable system.
In comparison with the relay device 200 according to the second example embodiment, the relay device 400 includes reflecting elements 411 and 412. The reflecting element 411 reflects only a C-band monitoring signal having a wavelength of λsvC-U in the uplink channel, and the reflecting element 412 reflects only a C-band monitoring signal having a wavelength of λsvC-D in the downlink channel. Signal loopback circuits 421 and 422 with such a configuration loop back only C-band monitoring signals to the channels on the opposite sides.
A pump laser diode that excites an amplification medium is sometimes shared by the C-band optical amplifier 121 and the L-band optical amplifier 122. For this reason, for example, when an output of pump light decreases due to malfunction of the pump laser diode of the uplink channel, outputs of both of the optical amplifiers 121 and 122 decrease. Thus, looping back only a C-band monitoring signal in the uplink channel enables malfunction of the pump laser diode in the uplink channel to be monitored by a land device in the downlink channel. Similarly, malfunction of a pump laser diode in the downlink channel is detected as well. Note that the signal loopback circuits 421 and 422 may loop back only an L-band monitoring signal to the channel on the opposite side, and malfunction of a pump laser diode may be monitored by an L-band monitoring signal.
In this manner, the relay device 400 according to the fourth example embodiment can relay an optical signal having a plurality of wavelength bands, and can loop back a monitoring signal. At the signal loopback circuits 421 and 422, the relay device 400 loops back the monitoring signal of one of the C-band and the L-band. Then, land devices can detect malfunction of the relay device 400 due to malfunction of pump laser diodes by the monitoring signals. It is sufficient that the signal loopback circuits 421 and 422 reflect monitoring signals having only one of the C-band and the L-band, and thus, configurations of the signal loopback circuits can be simplified as compared with the relay devices according to the first to third example embodiments.

Fifth Example Embodiment

FIG. 6 is a block diagram illustrating a configuration example of a relay device 500 according to a fifth example embodiment of the present invention. The relay device 500 includes a signal loopback circuit 511. The relay device 500 includes a configuration in which the reflecting elements 313 and 315 of the relay device 300 according to the third example embodiment are replaced with the reflecting elements 512 and 513 that reflect only the monitoring signal of the C-band or the L-band, similarly to the fourth example embodiment.
Similarly to the relay device 400 according to the fourth example embodiment, the relay device 500 loops back only the monitoring signal of one of the C-band and the L-band, and thereby, can notify a land device of malfunction of the relay device 500 due to malfunction of a pump laser diode. It is sufficient that the signal loopback circuit 511 reflects the monitoring signal of one of the C-band and the L-band, and thus, a configuration of the signal loopback circuit can be simplified as compared with the relay device according to the first to third example embodiments.

Sixth Example Embodiment

The advantageous effect of the present invention is also achieved by a signal loopback circuit of a sixth example embodiment described below. In the following, with reference to FIG. 5, the correspondence to the names or reference symbols of the respective elements according to the fourth example embodiment is written in parentheses. In other words, the signal loopback circuit (421) is a signal loopback circuit that makes connection between the channel of a first direction (UP) and the channel of a second direction (DOWN) through which an optical signal of a first wavelength band (C-band) and an optical signal of a second wavelength band (L-band) are transmitted.
The first coupler (241) splits an optical signal in the channel of the first direction. The first filter (411) extracts, from the optical signal split by the first coupler, at least one of a monitoring signal of the first wavelength band and a monitoring signal of the second wavelength band that are used in the channel of the first direction. The second coupler (243) combines, to the channel of second direction, the monitoring signal (λsvC-U or λsvL-U) extracted by the first filter.
The signal loopback circuit with such a configuration also enables a monitoring signal to be looped back in the relay device that relays an optical signal having a plurality of wavelength bands.
Although the invention of the present patent application is described above with reference to the example embodiments, the invention of the present patent application is not limited to the above-described example embodiments.
The present patent application claims priority based on Japanese Patent Application No. 2016-067181 filed on March 30, 2016 .

Reference signs List

100, 200, 300, 400, 500, 900: Relay device
111 to 114: Multiplexing-demultiplexing device
121 to 124, 901, 902: Optical amplifier
131 to 134, 211, 212, 311, 421, 511, 903, 904: Signal loopback circuit
141, 151, 241, 312: Coupler
142, 152, 242, 313, 315, 411, 412, 512: Reflecting element

Claims

A signal loopback circuit for making connection between a first channel of a first direction and a second channel of a second direction, the first direction being different from the second direction, wherein an optical signal of a first wavelength band and an optical signal of a second wavelength band are configured to be transmitted through each of the first and second channels, the second wavelength band being different from the first wavelength band, the signal loopback circuit comprising:
a first coupler (141) configured to split the optical signal of the first wavelength band in the first channel of the first direction;

a first filter (142) in the first channel of the first direction and configured to extract, from the optical signal of the first wavelength band split by the first coupler (141), a monitoring signal of the first wavelength band;

a second coupler (143) configured to combine, to the second channel of the second direction, the monitoring signal of the first wavelength band extracted by the first filter (142);

a third coupler (151) configured to split the optical signal of the second wavelength band in the first channel of the first direction;

a second filter (152) in the first channel of the first direction and configured to extract, from the optical signal of the second wavelength band split by the third coupler (151), a monitoring signal of the second wavelength band; and

a fourth coupler (153) configured to combine, to the second channel of the second direction, the monitoring signal of the second wavelength band extracted by the second filter (152).
The signal loopback circuit according to claim 1, further comprising:
a fifth coupler configured to split the optical signal of the first wavelength band in the second channel of the second direction;

a third filter in the second channel of the second direction and configured to extract, from the optical signal of the first wavelength band split by the fifth coupler, a monitoring signal of the first wavelength band;

a sixth coupler configured to combine, to the first channel of the first direction, the monitoring signal of the first wavelength band extracted by the third filter;

a seventh coupler configured to split the optical signal of the second wavelength band in the second channel of the second direction;

a fourth filter in the second channel of the second direction and configured to extract, from the optical signal of the second wavelength band split by the seventh coupler, a monitoring signal of the second wavelength band; and

an eighth coupler configured to combine, to the first channel of the first direction, the monitoring signal of the second wavelength band extracted by the fourth filter.
A relay device comprising:
a first multiplexing-demultiplexing device (111) configured to separate a wavelength-multiplexed optical signal propagating in the first direction, into an optical signal of the first wavelength band and an optical signal of the second wavelength band;

a second multiplexing-demultiplexing device (112) configured to combine the optical signal of the first wavelength band and the optical signal of the second wavelength band separated by the first multiplexing-demultiplexing device (111);

a third multiplexing-demultiplexing device (113) configured to separate a wavelength-multiplexed optical signal propagating in the second direction, into an optical signal of the first wavelength band and an optical signal of the second wavelength band;

a fourth multiplexing-demultiplexing device (114) configured to combine the optical signal of the first wavelength band and the optical signal of the second wavelength band separated by the third multiplexing-demultiplexing device (113); and

the signal loopback circuit according to claim 1 or 2.
The relay device according to claim 3, further comprising:
with respect to the first channel of the first direction,

optical amplifiers (121, 122) configured to amplify the optical signal of the first wavelength band and the optical signal of the second wavelength band separated by the first multiplexing-demultiplexing device (111), and

with respect to the second channel of the second direction,

optical amplifiers (123, 124) configured to amplify the optical signal of the first wavelength band and the optical signal of the second wavelength band separated by the third multiplexing-demultiplexing device (113).
The relay device according to claim 4, wherein the first coupler (141) and the third coupler (151) are arranged on an output side of the optical amplifiers (121, 122) configured to amplify the optical signal of the first wavelength band and the optical signal of the second wavelength band, respectively, separated by the first multiplexing-demultiplexing device (111).
A signal loopback method for making connection between a first channel of a first direction and a second channel of a second direction, the first direction being different from the second direction, wherein an optical signal of a first wavelength band and an optical signal of a second wavelength band are transmitted through each of the first and second channels, the second wavelength band being different from the first wavelength band, the signal loopback method comprising:
splitting the optical signal of the first wavelength in the first channel of the first direction;

extracting, from the split optical signal of the first wavelength, a monitoring signal of the first wavelength band that is used in the first channel of the first direction; and

combining the extracted monitoring signal of the first wavelength to the second channel of the second direction;

splitting the optical signal of the second wavelength in the first channel of the first direction;

extracting, from the split optical signal of the second wavelength, a monitoring signal of the second wavelength band that is used in the first channel of the first direction; and

combining the extracted monitoring signal of the secon wavelength to the second channel of the second direction.